1. Field of the Invention
The present invention relates to image forming apparatus having a double-sided image forming function, more particularly to the control of media transport speed in such apparatus.
2. Description of the Related Art
The media transport path in a conventional image forming apparatus having a double-sided image forming function is illustrated in FIG. 18. The media transport path in this apparatus, which is described in Japanese Unexamined Patent Application Publication No. 11-208962, begins in a feeding unit 1. Driven by a driving system not shown in the drawing, the feeding unit 1 feeds paper or other recording media from a cassette 2 toward a feed sensor 3 and a timing adjustment unit 4. As the recording medium leaves the timing adjustment unit 4, its thickness is sensed by a media thickness sensor 5 using, for example, a sensing method disclosed in Japanese Unexamined Patent Application Publication No. 10-31028, and a media thickness assessment module 6 is informed of the result. The recording medium then enters an image forming unit 7 in which an image is formed on one side of the recording medium by an electrophotographic process.
Next, the recording medium travels to a fuser 8 that applies heat and pressure to fuse the image onto the recording medium. The media thickness assessment module 6 indicates the thickness of the recording media to a fusing temperature control module, not shown in the drawing, that selects a fusing temperature suitable for the indicated thickness and controls the fuser 8 so as to bring the fusing temperature to the selected temperature.
The recording medium, carrying the fused image, now enters a delivery path 9 that carries it to a pair of delivery and reversing rollers 10 in a delivery unit 11. For one-sided image formation, also referred to as one-sided printing, the delivery and reversing rollers 10 deliver the recording medium from the delivery unit 11 to the exterior of the apparatus, completing the image forming process. For double-sided printing, a media reversing unit 13 including the delivery and reversing rollers 10 and a position sensor 12 sends the recording medium back into the image forming apparatus. Specifically, at a timing triggered by the position sensor 12, the direction of rotation of the delivery and reversing rollers 10 is reversed, reversing the transport direction of the recording medium. The recording medium is then carried into a return path 14 that branches away from the delivery path 9 so that the recording medium is in effect turned over.
While moving through the image forming unit 7 and on toward the delivery unit 11, and while being delivered, the recording medium travels at a predetermined speed V1. While moving in reverse, from the delivery and reversing rollers 10 back to the return path 14, the recording medium travels at a speed V2 faster than speed V1.
The return path 14 includes a transport sensor 15 and three pairs of refeeding rollers 16, 17, 18, which are driven and controlled so as to feed the recording medium to the timing adjustment unit 4 again. During this refeeding process, the recording medium continues to travel at the faster speed V2.
From the return path 14, the recording medium is fed through the timing adjustment unit 4 into the image forming unit 7 again, and another image is formed on the reverse side of the recording medium. This image is also fused by the fuser 8; then the recording medium is carried on the delivery path 9 to the delivery unit 11 and delivered to the exterior of the apparatus by the delivery and reversing rollers 10, completing the double-sided image forming process.
With increasing awareness of environmental issues and energy conservation, the double-sided printing function has come into wide use, and there is a growing need for image forming apparatus capable of double-sided printing on various different types of media. There is furthermore a rising expectation of faster printing speeds, and media transport speeds in image forming apparatus have accordingly increased significantly. In order to enable high-speed double-sided printing, the return transport speed (V2) must be considerably faster than the transport speed (V1) in the image forming unit. In the conventional apparatus, the return transport speed V2 has a fixed value independent of the type of recording media.
The demand for faster printing speed is matched by a rising demand for more compact apparatus, so the space available for accommodating additional functions such as double-sided printing has become extremely small. Therefore, when a double-sided printing function is present, the return path tends to include tight curves. The recording medium must negotiate these tight curves at high speed, so if the printing medium is thick and the driving motor does not have sufficiently high torque, there is a risk of transport failure due to the increased medium transport load. This type of transport failure can be prevented by using a large motor with high torque, but then the size and manufacturing cost of the apparatus are increased.